A magnetic disk has a disk (platter), and a magnetic recording layer is formed on the disk. As shown in FIG. 5A, a magnetic recording layer 55 is divided into a plurality of pieces of concentric storage area 56. Each storage area 56 is called a track. This track 56 is further divided into a plurality of pieces in the direction of rotation. Such divided region 57 is called a sector.
By using magnetism which occurs from a head 58 shown in FIG. 5B, information is written in the magnetic recording layer 55 of such magnetic disk. Also, by the head 58, information is read from the magnetic recording layer 55.
Meanwhile, in a magnetic disk, there is a tendency of increasingly higher track density (TPI (Track per Inch)) in which the track 56 becomes narrower in response to the demand of miniaturization and higher memory capacity. For this reason, when information is written in a sector 57 (track 56) by using magnetism from the head 58, a leakage flux from the head 58 tends to easily affect a track 56 (adjacent track) adjacent to the track 56 of the writing target. Specifically, when information is written into a certain track 56 a plurality of times, there occurs a phenomenon in which information stored (recorded) in an adjacent track 56 adjacent to that track 56 is demagnetized or degaussed, that is, adjacent track interference (Adjacent Track Interference (ATI)).
In order to prevent a situation in which information (data) recorded in an adjacent track is destroyed by this Adjacent Track Interference (ATI) (data corruption problem), there is a case where a refreshment operation is carried out to the adjacent track. A refreshment operation is an operation in which information recorded in a track is read once, and, in addition, the read information is written in the same track again.
In order to prevent the data corruption problem by Adjacent Track Interference (ATI), Japanese Patent Application Laid-Open No. 2008-257782 discloses the following method. That is, a magnetic disk apparatus indicated in this document monitors the number of times of writing in each sector in a magnetic disk. When it is detected that the number of times of writing in a sector exceeds a prescribed threshold value, the magnetic disk apparatus replaces the address corresponding to the sector with an address corresponding to another sector. By this, because a sector corresponding to the address of a large writing frequency is changed, phenomena where information is further written into a sector with a large writing frequency can be reduced. As a result, the magnetic disk apparatus can prevent information which is recorded in an adjacent track neighboring a sector with a large writing frequency from being damaged due to Adjacent Track Interference (ATI).
Japanese Patent Application Laid-Open No. 1988-81662 discloses a method to handle a problem caused by a bad track (bad sector) in which information cannot be written normally any more due to repeated writing.
In order to prevent the data corruption problem caused by Adjacent Track Interference (ATI), there is a case where a refreshment operation is carried out to a track being in danger of data corruption as described above. In this case, when a writing frequency in a track next to a track (in this description, it is supposed to be a track R) of a refreshment operation target is high, a refreshment operation is carried out repeatedly to the track R. Repeatedly carried out refreshment operation could lead to making the time needed for reading and writing information (data) from/to a magnetic disk long. In other words, a refreshment operation has a risk to deteriorate the performance of a magnetic disk apparatus equipped with a magnetic disk.